1) Field of the Invention
The present invention relates to a bandpass filter for use in a portable transceiver or the like in a frequency range of several hundred megahertz several gigahertz
2) Description of the Prior Art
As a bandpass filter used in a frequency range of several hundred megahertz--several gigahertz, there are one shown in FIG. 1 proposed by the inventors of the present invention and a helical filter as shown in FIG. 2. FIG. 1a is a front view, FIG. 1b is a side view and FIG. 1c is a rear view.
The bandpass filter shown in FIG. 1 has a construction as described below. A baked dielectric substrate 130 has substantially C-shaped conductive films 131 and 132 symmetrically arranged on its front surface and substantially C-shaped conductive films 133 and 134 symmetrically arranged on its back surface. The conductive films 131 and 132 are soldered with grounding terminals 135, and the conductive films 133 and 134 are soldered with external lead terminals 136. Then, this bandpass filter, excluding the projecting portions of the grounding terminals 135 and the external lead terminals 136, is immersed in an insulating resin bath, whereby the bandpass filter is wrapped up by a resin film.
Each of the conductive films 131 to 134 comprises two capacitor electrodes 131a and 131b, 132a and 132b, 133a and 133b, or 134a and 134b, and one inductor electrode 131c, 132c, 133c or 134c. The capacitor electrodes 131a, 131b, 132a and 132b are opposed to the capacitor electrodes 133a, 133b, 134a and 134b, respectively, across the dielectric substrate 130 and each opposed pair of capacitor electrodes has a capacitance determined by the dielectric constant and thickness of the dielectric substrate 130 and surface area where one of the capacitor electrodes is opposed to the other. On the other hand, the inductor electrodes 131c to 134c connect the capacitor electrodes 131a to 131b, 132a to 132b, 133a to 133b and 134a to 134b, respectively.
Each of the inductor electrodes 131c to 134c has a inductance. Therefore, a pair of conductive films opposed to each other across the dielectric substrate 130 constitutes an LC resonator. In FIG. 1, two such LC resonators on the dielectric substrate 130 are magnetically coupled with each other because the inductor electrodes 131c and 132c are closely positioned, thereby constituting a bandpass filter.
On the other hand, the helical filter shown in FIG. 2 has a construction wherein two helical resonators 140 are magnetically coupled with each other through cutouts 142. The cutouts 142 are provided by cutting out the opposed portions of the metal caps 141. Number 143 indicates an insulator and number 146 indicates a coil wound around the insulator 143.
In the bandpass filter shown in FIG. 1, shielding is necessary to prevent affection by outer electromagnetic field. The shielding, however, requires a shielding metal plate to be provided away from the conductive films 131 through 134 in a desired distance, which results in a large size bandpass filter.
On the other hand, in the helical filter shown in FIG. 2, the metal caps serve as shielding members and no separate shielding member is required. However, this filter is difficult to be compact and is not suitable for surface mounting.